Queen Elizabeth II, the world's oldest and longest-reigning monarch who turned 91 today, celebrated her birthday in a low-key fashion privately at her Windsor Castle home near here.
Buckingham Palace marked the occasion with the release of a photograph of the Queen as a 10-day-old old baby in the arms of her parents in May 1926.
The black and white photo from her christening was tweeted this morning as gun salutes rang out across London.
Clarence House, the Prince of Wales and wife Camilla - Duchess of Cornwall's official residence, shared another black and white picture of the Queen and a young Prince Charles from 1952.
Well-wishers have been using the hashtag #HappyBirthdayHerMajesty to tweet their birthday messages for the Queen.
The monarch, who has given herself the day off from formal engagements, is marking the milestone privately at her Windsor Castle home near London.
She became Britain's longest-reigning monarch, passing her great-great grandmother Queen Victoria's record in September 2015.
With the death of 88-year-old King Bhumibol Adulyadej of Thailand last year, the Queen is also now the world's longest reigning living monarch.
On February 6, the date she became Queen in 1952, she achieved another milestone, becoming the first British sovereign to reach a Sapphire Jubilee to mark 65 years of her reign.
The Queen has two birthdays, with April 21 marking the actual day she was born as Princess Elizabeth Alexandra Mary in 1926, the first child of the then Duke and Duchess of York.
The more celebratory birthday is marked in the middle of June, traditionally known as her "official" birthday, to be able to hold more public events during better weather in the UK.
The artillery 41-gun salute at Hyde Park and 62-gun salute at the Tower of London in the British capital and 21- gun salute at Windsor Great Park will be the only formal ceremony to mark her birthday today.
They are fired by the King's Troop Royal Horse Artillery and the Honourable Artillery Company.
This year the Queen's official birthday falls on June 17, which will be marked by the ceremonial Trooping of the Colour.
There was a major open air picnic outside Buckingham Palace last June to mark her 90th birthday.
Since hitting that milestone, the Queen has been gradually passing on more and more of her royal duties to younger members of the royal family - including her son and heir Prince Charles, and grandsons William and Harry.
However, she is still seen out and about at various events - most recently feeding a banana to an elephant at Whipsnade Zoo in Bedfordshire to mark the opening of a new centre for Asian elephants.
The Queen is spending the month of April at Windsor Castle, known as Easter Court.
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